1. Field Of The Invention
This invention relates to artificial joints and in particular to artificial joints of the ball and socket type.
2. Description Of The Prior Art
As is well known in the art, artificial hip and shoulder joints conventionally employ ball and socket articulations. The socket is embedded in one bony structure, for example, the pelvis for a hip reconstruction. The ball is attached to an arm composed of a neck and a stem or shaft, the stem or shaft being embedded in another bony structure, for example, the femur for a hip reconstruction.
Of critical importance to the success of such artificial joints is the ability to retain the ball within the socket, that is, the ability to avoid dislocations of the joint. A number of methods are known to minimize the occurence of dislocations.
In the most common method, referred to herein as the "semi-constrained" construction, the patient's own anatomy, i.e., his muscles, tendons and ligaments, are used to retain the ball within the socket. For this construction, a hemispherical socket typically is used which allows the ball and its attached arm the maximum amount of movement without contact of the arm with the edge of the socket. The surgeon, when installing such a semiconstrained joint, must align the ball and socket as closely as possible with the patient's natural anatomy so that the patient's movements do not tend to dislocate the ball from the joint. As a general proposition, such precise alignment is easiest the first time an artificial joint is placed in a patient. Subsequent reconstructions are much more difficult to align because of deterioration of anatomical landmarks as a result of the first operation, the healing process after the operation and changes in the anatomy caused by the presence of the artificial joint.
In order to increase the inherent stability against dislocation of such semi-constrained constructions, it has become conventional to add a cylindrical portion to the hemispherical socket to make it deeper. Although the ball is not physically constrained by the socket by this adjustment, the ball does have further to travel than if just a hemisphere had been used and thus some reduction in the propensity towards dislocation is achieved. Ball and socket joints of this type generally provide an arc or range of motion of approximately 115.degree. when a 28 mm diameter sphere is used and the socket is made a few millimeters deeper than a hemisphere. Larger ranges of motion can be obtained by keeping the size of the arm attached to the ball constant and increasing the diameter of the ball. In this way, the angular extent of the arm relative to the ball becomes smaller. In the limit, if the ball could be made progressively larger and larger, a range of motion of 180.degree. could be achieved. In practice, however, the largest sphere in common use in artificial joints, and in particular artificial hip joints, has a diameter of 32 mm and provides a range of motion of approximately 120.degree.. It should be noted however, that such larger sphere sizes are not universally favored because frictional torque increases with diameter.
A recent study by the Mayo clinic, which appeared in December, 1982 edition of The Journal of Bone and Joint Surgery, reported a dislocation frequency of 3.2% for 10,500 hip joint implant procedures using the semiconstrained construction. Such dislocations essentially make the patient immobile and can necessitate a second operation. As discussed above, the critical alignment required for the semi-constrained construction is even more difficult to achieve when a second implantation is performed. Accordingly, even higher dislocation frequencies are encountered for second and subsequent implantations.
An alternative to the semi-constrained construction is the construction wherein the ball is physically constrained within the socket. In this construction, a spherically-shaped bearing surrounds the ball and serves as the socket. The bearing is attached to a fixation element which is embedded in, for example, the patient's pelvic bone. The bearing encompasses more than one-half of the ball and thus constrains the ball and its attached arm from dislocation.
The bearing is typically made from plastic, such as ultra-high molecular weight polyethylene (UHMWPE), or metal. For plastic bearings, the ball and bearing are usually assembled by forcing the bearing over the ball. The more of the ball which is encompassed by the bearing, the greater the required assembly force, and the greater the constraining force to prevent postoperative dislocation of the joint. In addition, the more that the bearing encompasses the ball, the smaller the range of motion for the ball prior to contact of the bearing with the arm attached to the ball.
An example of a constrained artificial joint employing a plastic bearing is shown in Noiles, U.S. Pat. No. 3,996,625. As can be seen in FIG. 1 of this patent, a plastic bearing 17 fitted with a metal reinforcing band (un-numbered) extends beyond the diameter of ball 24 so as to physically constrain the ball within the bearing. The bearing itself is attached to fixation element 12. The metal reinforcing band is assembled over the lip of the opening of bearing 17 after that bearing has been forced over sphere 24. The reinforcing band increases the force required to dislocate the joint. In practice, the design shown in FIG. 1 of U.S. Pat. No. 3,996,625 has been found to provide a range of motion of approximately 85.degree. when a sphere diameter of 28 mm is used and to resist direct dislocating forces of several hundred pounds.
For constrained constructions such as that shown in U.S. Pat. No. 3,996,625, it has been found in use that a dislocating force is created when the neck of the arm attached to the ball impinges on the rim of the bearing. Because of the leverage associated with the arm and the long bone of the patient to which it is attached, e.g., the patient's femur, the dislocating force produced when the neck contacts the rim of the bearing can be considerable. For example, a force on the order of 25 pounds applied to a patient's leg can produce a dislocating force of over several hundred pounds because of the leverages involved. This type of dislocation force can be avoided by geometrically aligning the artificial joint with the patient's anatomy so that the neck does not come in contact with the rim of the bearing during normal motion of the patient's limb. That is, the leverage based dislocation forces can be avoided in the same way as dislocations are avoided in the semi-constrained construction, i.e., through precise alignment of the artificial joint with the natural anatomy of the patient. Unfortunately, as is apparent from the geometry of the situation, the more the socket bearing encompasses the ball, the greater the restraining force on the ball, but at the same time the less the range of motion prior to the neck impinging upon the edge of the bearing to create undesired leverage. In practice, artificial hips having the construction shown in U.S. Pat. No. 3,996,625 have been found to suffer dislocation due to the leverage effect in fewer than 0.5% of the implantations performed. This is significantly better than the 3.5% dislocation frequency reported in the Mayo clinic study discussed above, but an even lower dislocation frequency is obviously desirable.
A constrained construction using a metal socket bearing is shown in Noiles, U.S. Pat. No. Re. 28,895 (reissue of U.S. Pat. No. 3,848,272). This construction provides approximately a 90.degree. range of motion when the sphere diameter is 28 mm. In a practical sense, the metal bearing can be said to be non-dislocatable. The force required to extract the metal sphere from the enclosing metal socket bearing is more than several thousand pounds. Accordingly, in use, rather than the metal ball dislocating force the metal socket bearing, any overly severe dislocating leverage will cause the fixation element to be disrupted from the bone in which it has been embedded.
As a general proposition, metal balls in metal socket bearings are used in only a minority of joint reconstructions because the medical profession is not in agreement that a metal sphere in a metal bearing is as biologically acceptable as a metal sphere in a UHMWPE plastic bearing, even though clinical use over 15 years has failed to show the metal to metal joint to be inferior to a metal to plastic joint.
A third type of artificial ball and socket joint, referred to as an endoprosthesis, eliminates the fixation element associated with the socket and simply uses a ball surrounded by a plastic socket bearing in a spherical metal head, which head is placed in the patient's natural socket but not secured to bone. For this construction, the ball can rotate within the bearing up to the rim of the bearing (the bearing is greater than a hemisphere so as to be retained on the ball), and then the bearing and its attached head rotates in the patient's socket. As with the semi-constrained construction, anatomical alignment is used to avoid dislocations, in this case between the metal head and the natural socket.
In view of the foregoing, it is apparent that in semi-constrained and endoprosthesis hip joints, reconstructive geometry of the prosthetic components is critical in ensuring the stability of the prosthesis against dislocation. Moreover, in ball and socket constructions which constrain the elements against dislocation, the range of motion inherent in the prosthesis is reduced and thus because of the possibility of leverage type dislocations, similar demands are placed on the surgeon to establish the geometry of the reconstruction within rather narrow limits.
Accordingly, an object of this invention is to provide a ball and socket bearing for an artificial joint which constrains the joint from dislocating and at the same time provides a range of motion which is greater than that available in the constructions of the constrained type described above.
A further object of this invention is to provide a ball and socket joint which provides the surgeon with increased latitude in geometric positioning of the prosthetic components over those ball and socket joints presently available.
A further object is to provide a prosthetic ball and socket joint of increased inherent range of motion which is readily assembled and disassembled at the surgical site.